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Thread: Loss of power

  1. #1

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    My 1980 CX 500 Custom runs well except it won't accelerate past about 55 MPH. It idles well and runs pretty smooth. Out of gear it revs up well but when I get on the road the top speed in 5th gear is about 55. This started this spring. It was fine last summer. Does anyone have any idea what might be wrong with my bike?

  2. #2
    Senior Member Blue Fox's Avatar
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    Two things come to mind. Did you do any work on the carbs since it ran good? Installing the jets in the wrong towers will cause what you are experiencing.



    The other thing would be the high speed coil of the stator has given up. You might try the "White Wire Fix" just to see if it changes. Could also be the CDI giving up at high speeds. And probably a few more that others will post shortly.



    You can check the voltages on the blue (high speed) and white (low speed) coils in the stator with a multimeter. Set the meter for AC VOLTAGE, and measure the voltage from each wire. You can even have it running for the blue wire. You should be seeing approx 90 volts on the white, and 100+ on the blue. Remember, measure at the wire coming from the stator, not the CDI. The White wire fix simply connects the blue wire going into the CDI with the white coming from the stator. This provides marginally high enough voltage for the high speed section of the CDI.
    1983 GL650I, 1980 CX500, 1974 CB350F, 1973 CT90

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  3. #3
    Senior Member Allan's Avatar
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    Does the bike work normal until it hits 55 and then work bad? Or does it just have no power and tops out at 55?



    If it has no power and tops out at 55 it may be only running on one cylinder.

    If this is what is happening it could be as simple as a bad spark plug.
    ...and as the elders of our time choose to remain blind, let us rejoice and let us sing and dance and ring in the new. Hail Atlantis!

    1975 XL-100, 1978 CX500, 1980 CX500, 1981 CX500

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  5. #4

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    Quote Originally Posted by Allan View Post
    Does the bike work normal until it hits 55 and then work bad? Or does it just have no power and tops out at 55?



    If it has no power and tops out at 55 it may be only running on one cylinder.

    If this is what is happening it could be as simple as a bad spark plug.


    No it seems a little sluggish in acceleration from a standing start but not really bad in the first 4 gears but once you shift into 5th it's like hitting a wall at 55. I can get it faster if I stay in 4th it'll climb to 60 or so but shift to 5 and it actually slowly decreases speed to around 55. It's reall strange. It's running on both cylinders when I've checked it a idle. I tried replacing both plugs already no differencewhen I did that.

  6. #5

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    Quote Originally Posted by Blue Fox View Post
    Two things come to mind. Did you do any work on the carbs since it ran good? Installing the jets in the wrong towers will cause what you are experiencing.



    The other thing would be the high speed coil of the stator has given up. You might try the "White Wire Fix" just to see if it changes. Could also be the CDI giving up at high speeds. And probably a few more that others will post shortly.



    You can check the voltages on the blue (high speed) and white (low speed) coils in the stator with a multimeter. Set the meter for AC VOLTAGE, and measure the voltage from each wire. You can even have it running for the blue wire. You should be seeing approx 90 volts on the white, and 100+ on the blue. Remember, measure at the wire coming from the stator, not the CDI. The White wire fix simply connects the blue wire going into the CDI with the white coming from the stator. This provides marginally high enough voltage for the high speed section of the CDI.
    Thanks I'll look at those things tonight. I haven't done anything but an oil change when this started but I did put in new plugs once I got it out and found the problem. It sat in my garage from August until late March. I drained the gas too because i thought that might be the problem. It's hard to troubleshoot because it runs fine on the center stand. Seems to only be an issue under load at speed. It is a bit sluggish from a start but not significantly so until I shift to 5th gear?

  7. #6
    Senior Member stern-69's Avatar
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    Have you dont a compression test ? Only two things I can think of are bad rings or carb issues. If the compression seems ok on both cylinders, check the carb ballancing as it can make a huge difference in performance.
    1982 CX500C
    1979 GL1000

    For detailed service and repair info, see the CX WIKI Pages
    Rebuilding a carb, do it right with Larry's Carb Book


    Some people are like answering machines..... they ask but are incapable of listening

  8. #7

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    Quote Originally Posted by FADM Stern GNSF View Post
    Have you dont a compression test ? Only two things I can think of are bad rings or carb issues. If the compression seems ok on both cylinders, check the carb ballancing as it can make a huge difference in performance.
    I haven't checked the compression. I'm pretty sure the carbs are good. The bike only has 11,500 miles and I've changed the oil every spring so I hope the rings aren't bad.

  9. #8
    Senior Member stern-69's Avatar
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    Chances are it could be something as simple as a bit of gunk in the carbs (which is VERY possible if you ran really old gas through it). I rebuilt my carbs twice with little luck until I got and used Larry's book, and boy did that make a huge difference. Went from not being able to keep it running, backfiring and generally crappy to smooth and powerfull with +68MPG on the fuel useage. These bikes are pretty hardy units and were built well, but do have some finicky bits (like the carbs). Best thing when trying to solve a problem is elliminate all the really easy things (like freash gas, good battery, good plugs etc) as its surprising how bad a bike can run with something small not right.
    1982 CX500C
    1979 GL1000

    For detailed service and repair info, see the CX WIKI Pages
    Rebuilding a carb, do it right with Larry's Carb Book


    Some people are like answering machines..... they ask but are incapable of listening

  10. #9
    Senior Member Allan's Avatar
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    Have you checked the air box and air filter? Maybe a critter built a nest in there,,



    If there is not a problem with that,,I would try removing the spark plug wires one at a time when driving in 5th gear to see if both cylinders are firing under load, not sure where you are located,,this would best be done on a back road.
    ...and as the elders of our time choose to remain blind, let us rejoice and let us sing and dance and ring in the new. Hail Atlantis!

    1975 XL-100, 1978 CX500, 1980 CX500, 1981 CX500

  11. #10

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    Quote Originally Posted by FADM Stern GNSF View Post
    Chances are it could be something as simple as a bit of gunk in the carbs (which is VERY possible if you ran really old gas through it). I rebuilt my carbs twice with little luck until I got and used Larry's book, and boy did that make a huge difference. Went from not being able to keep it running, backfiring and generally crappy to smooth and powerfull with +68MPG on the fuel useage. These bikes are pretty hardy units and were built well, but do have some finicky bits (like the carbs). Best thing when trying to solve a problem is elliminate all the really easy things (like freash gas, good battery, good plugs etc) as its surprising how bad a bike can run with something small not right.
    I've been careful not to buy gasahol but I did leave it with gas from last year without stablizer over the winter. I usually crank and run it at least once or twice a month all winter but didn't do it this past winter. That makes me think it could be a fuel issue but I thought once I drained and refilled it the problem would go away. It didn't. So I replaced the plugs. No better.

    I have no expertise on doing carb work and the guy I used to take it to has closed up shop. He has done what little maintenance work it needed over the years I've owned the bike so I'm totally lost. I have ordered a manual for the bike which should arrive any time. I hope it has so troubleshooting tips. It's hard to find a problem that only shows up when the bike is on the road.

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